小降雨事件对土壤水分及植物水势空间差异性的影响

本文以云南省蒙自市断陷盆地高原面上典型封闭式岩溶洼地小流域为研究对象，研究小降雨事件对土壤水分及植物水势的影响。结果表明:（1）在研究区内小降雨事件一般只能补给10 cm以上的土壤，因此在旱季（强降雨事件发生前），土壤水分随着深度的增加整体呈变小的趋势；受洼地地形影响，整个土壤剖面（0 ~ 80 cm）的土壤水分存在从坡顶到洼地底部逐渐增加，苹果树叶水势逐渐升高的现象；受地质背景的影响，土石质坡地平均土壤水分比石质坡地土壤水分高2.67%，相对应的土石质坡地苹果树受干旱胁迫的程度要低于石质坡地。（2）通过观测对比发现，8天内12次的小降雨事件可以使0~10 cm土壤水分整体上略有升高，但并未能完全改变0~10 cm土壤水分洼地底部大于两侧坡地，而土石质坡地高于石质坡地的特征。（3）小降雨事件虽然只能补给0~10 cm的土壤水分，但由于坡地地区苹果树根系分布较浅（5~30 cm），部分浅层分布的苹果树根系已能吸收到水分，另一方面小降雨事件具有降温、增湿，减少太阳辐射的作用，可以减小苹果树蒸腾作用，从而降低苹果树叶水势，因此推测小雨事件可以明显减轻苹果树受干旱胁迫的程度 

Effects of light rainfall events on spatial variation of soil moisture and leaf water potential of apple tree (Malus pumila Mill.) in a karst graben basin, Yunnan Province

Karst graben basins are special type of karst landforms, which are widely distributed in southwestern Sichuan and Eastern Yunnan Province. In mountainous areas of karst graben basins, light rainfall takes a big proportion of annual precipitation with short occurrence. with short occurrence intervals. The purpose of this work is to study the spatial variations and the response processes of soil moisture and leaf water potential of apple tree (Malus pumila Mill.) to such light rainfall events. The results showed that,(1) the light rainfall events in the mountainous area can only supply the soil water to the maximum depth of 10 cm, so in the dry season (before heavy rainfall events), soil moisture declines with the depth in the vertical section. Due to the topography of the depression, soil moisture (0-80 cm) and leaf water potential of apple tree gradually increase along the slope direction from the top to the bottom of the depression. Because of geological background differences, the soil moisture in the west slope (soil-rocky slope) is 2.67% higher than that in the east slope (rocky slope), thus, the drought degree of apple trees on west slopes are less than that east slopes. (2) There were 12 times light rainfall events in 8 days which can make the surface soil moisture (0~10 cm) rise slightly, but failed to completely change the characteristics that the soil moisture in the bottom of the depression was higher than that on both slopes and the soil moisture of west slop was higher than that of east slope. (3) Although light rainfall events can only supply the soil moisture of 0-10 cm, some shallow apple roots can absorb water because of the shallow distribution (5-30 cm) of apple roots in sloping areas. On the other hand, light rainfall events could decrease air temperature, increase humidity and reduce the solar radiation, which could reduce the transpiration of apple tree and the water potential of apple leaves. It is indicated that the light rainfall events can significantly reduce the drought degree of apple trees.